DE-OS 39 18 510 describes a process for producing coatings resistant to the impact of stones by the use of a conventional stoving filler based on a polyester and a specially blocked isocyanate.
In connection with ever more exacting requirements as regards protection of the environment, demands have been made for low-emission lacquer systems. This has led to the development of water-dilutable stoving fillers based on a wide variety of binding agents.
EP-A-249 727, for instance, describes a hydrofiller based on a combination of epoxide resin-phosphoric or phosphonic ester, a water-dilutable polyester according to AT-PS 328 587 and water-compatible melamine resins. DE-OS 40 00 748 describes hydrofillers based on water-dilutable hydroxyfunctional polyurethane resins, optionally additional binding agents and water-compatible amine resins. DE-OS 38 13 866 describes hydrofillers consisting of water-dilutable polyurethane resin, water-dilutable epoxide resin-modified. polyester and optionally water-dilutable aminoplastic resin. DE-OS 38 05 629 describes a water-dilutable coating agent for producing coatings resistant to the impact of stones and based on carboxyl group-comprising polyesters or methacrylic copolymers and water-soluble blocked isocyanate prepolymers as well as optionally additional water-soluble amine resin.
All water-dilutable stoving fillers mentioned in the above specifications are, however, subject to the disadvantage that they cannot cope with all stresses due to the impact of stones such as occur in practice and do not therefore meet the criteria of the various test specifications applied by the various motor car manufacturers as regards resistance to the impact of stones. In particular, there are still in general deficiencies as regards the low-temperature resistance to the impact of stones, and the known water-based fillers are therefore still greatly in need of improvement. This is especially interesting in view of tendencies to change, as regards scattering materials applied during winter, from the conventional application of highway salt to increased application of grit or granulates.
Moreover, the known hydrofillers are not sufficiently resistant to overfiring, i.e. if the stoving temperatures appropriate to the system are exceeded, the characteristics of the layers of filler cease to be up to standard.